This invention relates to a vacuum valve.
Generally, in or to improve a breaking efficiency of a vacuum valve an arc control method of applying a magnetic field parallel to a vacuum arc generated between electrodes has been used to suppress the arc. A typical vacuum valve using the method is a longitudinal-flux-type vacuum valve. One of electrode structures of the longitudinal-flux-type vacuum valve is shown in FIG. 11. FIG. 11 shows a structure of a movable electrode. A structure of a stationary electrode is the same with the structure of the movable electrode and the stationary electrode is arranged to face the movable electrode for contacting thereto.
In FIG. 11, a round concave 6a is dug at a top of a movable conduction column 6B of copper. A ring-shaped reinforcing element 18 of stainless steel has a collar 18a of its lower portion and the collar 18a is engaged in the round concave 6a and brazed to it. A bush 14a of copper projecting from a center of a coil electrode 14 is inserted around the collar 18a and brazed with the collar 18a and the movable conduction column 6B.
Four arms 14b projects from the bush 14a in a radial pattern as to space 90xc2x0 each other around the bush 14a and in the direction perpendicular to the axial direction of the bush 14a. A base portion of an arc coil element 14c is brazed to each end of the arms 14b. A through hole 14d is bored at a top of the coil element 14c along the axial direction. A disk-shaped contact element 13 made of copper and having a center column is provided to the top of the coil element 14c and the center column of which is inserted into the top of the coil element 14c and is brazed thereto.
A disc-shaped electrode plate 2B made of copper with grooves cut in a radial pattern from the center to the circumference thereof is provided on the end of the reinforcing element 18 and that is brazed to the surfaces of the reinforcing element 18 and the contact element 13. A disc-shaped contact element 1A made of tungsten alloy with grooves cut in a radial pattern from the center to the circumference thereof and with a roundly chamfered outer edge is brazed to the electrode plate 2B.
In this vacuum valve having the electrode of the structure set forth above, a breaking current from the movable conduction column 6B to the contact element 1A mainly flows from the bush 14a through the arms 14b to the end of the coil element 14c of the coil electrode 14 and the small part of the current flows through the reinforcing element 18 to the electrode plate 2B.
The current flowing into the coil element 14c runs there half round so as to produce a longitudinal magnetic field and flows into the electrode plate 2B via the contact element 13 at the end of the coil element 14c and the lower surface of the electrode plate 2B. The current further runs through the upper surface of the electrode plate 2B and comes out from the contact element 1A. This current coming out from the contact element 1A flows into a contact element of the stationary electrode (not shown in FIG. 11) contacting to the surface of the contact element 1A and it runs through an electrode plate, a contact element and coil element of the stationary electrode and flows out into a stationary conduction column.
FIG. 12 shows a distribution of magnetic flux density between the electrodes produced by the coil electrode 14 (given at an area halfway between the movable and stationary electrodes when they are pulled apart). The longitudinal flux density between the electrodes is greatest at the center area of the electrode and it gradually lowers toward the circumference thereof. Here, in order to effectively suppress an eddy current to be generated by the coil electrode 14, slits are made in the electrode plate 2B and the contact element 1A. The coil electrode 14 is designed as the flux density to be larger even at the circumference of the electrode than a flux density Bcr which causes the lowest arc voltage to respective breaking currents.
By controlling the vacuum arc generated between the electrodes through this distribution of flux density, the breaking current that causes an arc concentration is greatly improved comparing to that to be caused under the condition without the magnetic field, and the breaking efficiency is also greatly improved. However, it does not mean that the arc concentration can be prevented to the indefinitely great current under the condition that the diameter of the electrode is defined. The arc concentration tends to occur in the center area of the electrode (in the neighborhood of an anode) in a strong magnetic field that is produced by a greater current than a critical value.
Additionally, as shown in FIG. 12 of the distribution of the magnetic flux density, the current density in the center area of the electrode has been detected very great even in the lower current region than the critical current. This tends to cause the current density in the center area to reach to the critical current density so that the arc shifts from its dispersed state to concentrated state and finally falls into non-breakable state.
In order to raise the critical current, it seems to be effective to unify the distribution of current density by changing the magnitude and the distribution of flux density to be adjusted. However, as to the intensity of the magnetic field, the inventors carried out current-breaking tests by using trial electrodes enabling to produce intensified magnetic fields but the result did not show the effectiveness.
Accordingly, the distribution improvement of flux density has been expected to be a solution for raising the critical current and there has been proposed several methods in line with this approach in the past. Here, one typical method for improving the distribution of flux density will be explained.
FIG. 13 shows curves of radial-direction distribution of flux density between the electrodes, which is cited from the paper (IEEE Transs. on Power Delivery, Vol. PWTD-1, No.4, October 1986) presented by the inventors. These curves show that, although the distribution of flux density differs according to the gap distance between the electrodes, the maximum value of flux density always appears at the circumferential side of the electrode. However, the maximum density in the radial direction appears at around 55% point of the radius 28.5 mm of the electrode and it is out of the scope of the distribution characteristic of flux density proposed by this invention. Further, the conventional distribution characteristic of flux density can not effectively disperse the arc generated between the electrodes to their circumferential areas.
There are three kinds of method known which can lower the flux density in the center area of the electrode.
(1) One of which is a method of producing a reverse magnetic field by an eddy current flowing the electrode plate and contact element by not cutting the slits in the electrode plate 2B and the contact element 1A.
(2) Other method is that provides an other coil electrode for producing the reverse magnetic field in the center area of the electrode.
(3) The third method is that brings the coil electrodes 14 of the movable side and the stationary side closer as possible.
Japanese Laid Open Application PS57-212719 discloses an electrode structure using the method (1). FIG. 14(a) shows a distribution of flux density of this electrode and FIG. 14(b) shows the structure of the electrode. A coil electrode 11 is joined to an end of a movable conduction column 6C and a join port 15 is made therein and a spacer 18 is joined in the center area thereof. An electrode plate 12 is joined to the coil electrode 11 via the join port 15 and the spacer 18. A field adjust plate 36 of pure copper is buried in a surface 35 of the electrode plate 12 so as the reverse magnetic field to be produced by the eddy current generated by this field adjust plate 36. A contact element 37 is joined on the upper surface of the field adjust plate 36.
The distribution of flux density produced by this vacuum valve of the structure is shown by curved line F2 in FIG. 14(a). In FIG. 14(a), dotted line F1 shows a distribution of flux density produced by an electrode having no such a field adjust plate like the plate 36. As can be seen from FIG. 14(a), the maximum density of the flux comes to appear at the circumferential area by the reverse current generated by the field adjust plate 36, but the radial position of the maximum density is about 40% of the radius of the electrode and it is out of the scope of this invention.
Although it does not aim to improve the distribution of flux density, Japanese Patent Publication PH4-3611 shows an electrode which produces the similar distribution of flux density. FIG. 15 shows the structure of the electrode and the characteristic of the distribution of flux density produced by the electrode. In this structure, when a coil 31 provided at an external place of an electrode 32 for producing a magnetic field is energized, the distribution of flux density of the electrode 32 becomes like a curved line G2 by an eddy current generated by a contact element 1B and the point of the maximum flux density appears at the circumference of the electrode 32. In this FIG. 15, a dotted curve G1 shows a distribution characteristic of flux density produced solely by the coil 31.
It is impossible to conclude since there is not shown concrete numerical values in its publication. But judging from the position giving the maximum value and the ratio of the density values between the maximum point and the center area, it seems to fall in to the scope of this invention.
However, judging from the description of the publication it is out of the scope of this invention because the description expresses that the flux density of the center area of the electrode was greatly lowered and the longitudinal magnetic field did not effectively affect and further, the flux density of the center area of the electrode is apparently lower than the flux density aimed by this invention. Furthermore, as can be seen from FIG. 15, the flux density of the circumferential end of the electrode is drawn to near zero and it can not satisfy the criteria of the condition as the conventional art corresponding to this invention (the flux density should be equal to or greater than 2 mT/KA at the circumferential end of the electrode).
Japanese Laid Open Application PS57-20206 discloses an electrode structure using the method (2) set forth above. FIG. 16 shows a characteristic of a distribution of flux density between electrodes using the method (2). In the distribution of flux density shown in FIG. 16, the position giving the maximum flux density seems to fall in to the scope of this invention. However, the flux density produced by a coil for generating magnetic field at the center area of the electrode is reverse and the value at the center area of the electrode differs from that required by this invention.
Several other proposals which disclose electrode structures for producing reverse magnetic field at the center area thereof are found. However these proposed structures differ from this invention because they all produce the magnetic field of the reverse direction at the center area of the electrodes.
Japanese Patent Publication PH2-30132 discloses an electrode structure using the method (3). FIG. 17 shows a distribution characteristic of flux density between electrodes using the method (3). Compared to the method (2), the flux density at the center area of the electrode is not minus and the radial position giving the maximum flux density seems to fall in to the scope of this invention. However, the maximum value of flux density is about 2.5 times greater than that of given at the radial position 40% from the center of the electrode and this characteristic is out of the scope of this invention. Further, an axial flux density distribution from the center to the circumference of the electrode is not monotonously increasing and at this point, it differs from this invention.
In the conventional vacuum valve, as set forth above, there is drawback that the arc generated tends to concentrate to the center area of the anode since the flux density of the center area of the electrode is too great or too small. Additionally, since the arc tends to concentrate at one area, the energy density becomes too high when the arc flows into the surface of the anode. Therefore, the surface of the electrode sustains great heat damages and the temperature of the surface is kept high during the current breaking, and this makes the current breaking unable.
One of the objects of this invention is to provide a vacuum valve which can raise the critical current that starts the arc concentration by means of unifying the flux density along the surface of the electrode.
Other object of this invention is to provide a vacuum valve which improves the efficiency of current breaking by means of making the arc concentrate to plural points on the circumferential area of the electrode so as to decrease the current density at the area where the arc current is concentrating even if the current density on the surface of the electrode becomes higher than the critical current value and begins to concentrate.
Generally, voltage drop Vcolm in an arc column relates to axial flux density Bz and current density Jz as expressed below.
Vcolmxe2x88x9dJz/Bzxe2x80x83xe2x80x83(1)
Therefore, if the flux density is high at the center area of an electrode, the voltage drop Vcolm tends to decrease even when a current of the same density flows. As the degree of the voltage drop Vcolm between the electrodes is constant on the whole surface of the electrode and balances with the Vcolm on the circumferential area of the electrode, the current density Jz becomes high at the center area where the flux density is also high. This results in, in the conventional art, that the current density between the electrodes becomes high in the center area thereof as same as the flux density and it gradually decreases toward the circumference thereof as shown in FIG. 12.
In order to unify the current density over the surface of the electrode, it is necessary to suppress the current density in the center area of the electrode and to increase the current density at the circumferential area thereof. Accordingly, in order to suppress the current density in the center area of the electrode, this invention proposes to lower the axial flux density in the center area and to make the voltage drop large in the arc column at the center of the electrode so as to make the current flow uneasily. By using this method, the flux density of the circumferential area of the electrode becomes relatively high compared to the flux density in the center area thereof so as to make the voltage drop in the arc column become small and the current flow easily. The vacuum arc is carried its current mainly by an electron flow and, in the region of flux density intensified, Lamor radius is small and the arc is effectively captured by the magnetic line of force. As a result, the current comes to steadily flow in the circumferential area of the electrode which producing strong magnetic field and it becomes possible to unify the current density between the electrodes compared to the conventional art.
Further, in order to prevent the arc from concentrating to the center area of the electrode when the current increases greater than the critical current value, plural areas where the current density becomes slightly high are provided by means of changing the strength of flux density along the circumferential direction of the electrode, and this makes the arc concentrate to the plural areas respectively and decentralized. As a result, the arc comes to concentrate at plural areas and the current density of each area can be suppressed lower than that of the conventional art where the arc tends to concentrate at one spot.
In order to achieve this object, a first aspect of the invention is a vacuum valve characterized by producing an axial magnetic field parallel to an arc generated between a movable and a stationary electrodes facing each other, and being adjusted a magnitude of an axial flux density between the electrodes to increase gradually from a center area toward a circumferential area of each electrode, a point giving a maximum value (Bp) of the axial flux density to appear at a location equal to or outer than 70% of a radius from the center of each electrode, and the maximum value (Bp) in a radial line from the center to the circumferential end as to be 1.4 to 2.4 times greater than a flux density of the center (Bct) of each electrode.
An invention claimed in claim 12 is a vacuum valve for producing an axial magnetic field parallel to an arc generated between a movable and a stationary electrodes facing each other, and adjusting a magnitude of an axial flux density between the electrodes to increase gradually from a center area toward a circumferential area of each electrode, a point giving a maximum value (Bp) of the axial flux density to appear at a location equal to or outer than 70% of a radius from the center of each electrode, and the maximum value (Bp) of the axial flux density in a radial line from the center to a circumferential end to be 1.05 to 2.16 times greater than an axial flux density (Bcr) which is produced when an arc voltage becomes the lowest (Vmin) according to a relationship between the arc voltage and the axial flux density, where the arc voltage is defined by the radius of each electrode and a breaking current.
An invention claimed in claim 13 is a vacuum valve in which an axial flux density of the circumferential end of each electrode to be equal to or greater than 2 mT/KA.
An invention claimed in claim 14 is a vacuum valve according to claim 11 to 13 characterized by the flux density (Bct) of the center of each electrode being adjusted as to be 0.75 to 0.9 times greater than a flux density (Bcr) which giving the lowest arc voltage (Vmin).
An invention claimed in claim 15 is a vacuum vale according to claim 1 to 4 characterized by a radial position producing the flux density (Bcr) according to the lowest arc voltage being adjusted to locate within 20% to 40% area of the radius of each electrode.
An invention claimed in claim 16 is a vacuum valve according to claims 1 to 15 characterized by proving plural portions on a circular line passing through a radial point on each electrode at which the maximum flux density (Bp) is to be produced, where flux densities are to be 0.6 to 0.9 times greater than the greatest value (Bmax) among the maximum flux densities (Bp).
An invention claimed in claim 17 is a vacuum valve according to claim 16 characterized by a distribution of an axial flux density along the circular line passing through the radial point on each electrode at which the maximum flux density (Bp) is produced being adjusted as to have more than half portion of the circular line where, when the greatest value of flux density is set as Bmax and the smallest value of flux density as Bmin among the maximum flux densities (Bp), the flux density to be produced is greater than (Bmax+Bmin)/2.
An invention claimed in claim 18 is a vacuum valve having a pair of electrodes each of which is accommodated in a vacuum chamber and joined to a conduction column for electrical connection with an external element and both of which are facing each other for contacting, and characterized by having a contact element on a surface of each electrode, the contact element having a graded characteristic that a degree of a cathode voltage drop continuously or gradually reduces from the center to the circumference thereof.
An invention claimed in claim 19 is a vacuum valve according to claim 18 characterized by the contact element being made of copper-chrome (CuCr), and a weight percent of the chrome therein being adjusted to increase gradually from the center area to the outer side comprising a vacuum chamber, a conduction column in the vacuum chamber, a pair of electrodes each of which is accommodated in the vacuum chamber and joined to the conduction column for electrical connection with an external element and both of which are facing each other for contacting, a central conduction coil for producing the longitudinal magnetic field provided behind a central portion of each electrode, a plurality of peripheral conduction coils having a similar characteristic to the central conduction coil behind each electrode for producing the longitudinal magnetic field, and a current restraining member inserted between a top surface of the central conduction coil and each electrode.
An invention claimed in claim 21 is a vacuum valve comprising a vacuum chamber, a conduction column in the vacuum chamber, a pair of electrodes each of which is accommodated in the vacuum chamber and joined to the conduction column for electrical connection with an external element and both of which are facing each other for contacting, a plurality of conduction studs provided at peripheral positions in a rear side of each electrode, a plurality of magnetic members respectively provided adjacent to each conduction stud for being magnetized by a magnetic field produced by each conduction stud.